Warming temperatures caused by climate change are predicted to vary temporally and spatially. For mid‐ and high‐latitude reptiles, the seasonal variation in warming temperatures experienced by embryos and hatchlings may determine offspring fitness, yet this has remained largely unexplored. To evaluate the independent and interactive influence of seasonal variation in warming temperatures on embryonic and hatchling development, we incubated eggs and reared hatchlings of a cold‐climate oviparous ectothermic species, the Heilongjiang grass lizard (Takydromus amurensis), following a 2 × 2 factorial design (present climate versus warming climate for embryos × present climate versus warming climate for hatchlings). We then evaluated embryonic and hatchling development, including hatching success, incubation period, initial hatchling body size, hatchling metabolic rate, growth rate and survival in the mesocosms. We found that warming temperatures shortened the incubation period and produced hatchlings with higher survival rates than those incubated under the present climate conditions. Similarly, hatchlings reared under a warming climate had similar growth rates and resting metabolic rates, but higher survival rates than those reared under the present climate. Hatchlings that experienced both warming incubation and warming growth conditions had the highest survival rates. This study revealed that moderate warming temperatures (Representative Concentration Pathway, RCP 4.5, 1.1–2.6°C) experienced by embryos and hatchlings interact to benefit hatchling fitness in cold‐climate oviparous ectotherms. Our study also highlighted the importance of integrating seasonal variation in warming temperatures when evaluating the responses to climate warming in multiple developmental stages in oviparous ectotherms.